Optical Power Transfer Research Articles

Optical power transfer and communication methods for wireless implantable sensing platforms.

Ultrasmall scale implants have recently attracted focus as valuable tools for monitoring both acute and chronic diseases. Semiconductor optical technologies are the key to miniaturizing these devices to the long-sought sub-mm scale, which will enable long-term use of these devices for medical applications. This can also enable the use of multiple implantable devices concurrently to form a true body area network of sensors. We demonstrate optical power transfer techniques and methods to effectively harness this power for implantable devices. Furthermore, we also present methods for optical data transfer from such implants. Simultaneous use of these technologies can result in miniaturized sensing platforms that can allow for large-scale use of such systems in real world applications.

Imaging and steering an optical wireless nanoantenna link.

Optical nanoantennas tailor the transmission and reception of optical signals. Owing to their capacity to control the direction and angular distribution of optical radiation over a broad spectral range, nanoantennas are promising components for optical communication in nanocircuits. Here we measure wireless optical power transfer between plasmonic nanoantennas in the far-field and demonstrate changeable signal routing to different nanoscopic receivers via beamsteering. We image the radiation pattern of single-optical nanoantennas using a photoluminescence technique, which allows mapping of the unperturbed intensity distribution around plasmonic structures. We quantify the distance dependence of the power transmission between transmitter and receiver by deterministically positioning nanoscopic fluorescent receivers around the transmitting nanoantenna. By adjusting the wavefront of the optical field incident on the transmitter, we achieve directional control of the transmitted radiation over a broad range of 29°. This enables wireless power transfer from one transmitter to different receivers.

Development and characterisation of laser power converters for optical power transfer applications

Interest in single-junction photovoltaic devices has been renewed in recent years due to their potential to produce very high optical to electrical conversion efficiencies under monochromatic illumination. A single-junction photovoltaic converter was developed for monochromatic radiation (a laser power converter (LPC)) to specifically convert monochromatic light at a wavelength of 1.55 µm (which is eye and skin safe up to a power density of 1 kW/m2) into electrical energy. Such devices find use in space-based laser power transfer, as well as fibre optic power delivery and the remote powering of subcutaneous equipment and other electrical devices. The LPC design is based on lattice-matched InGaAsP/InP, which includes elements for photon recycling and efficient carrier extraction. Here, the design process is reported based on computer simulated characteristics, and compare them to experimental data from both laboratory and field tests. The LPC demonstrates a conversion efficiency of 45% ± 1% under illumination at 1.55 µm and 1 kW/m2, and a conversion efficiency of 13.3% under AM1.5 solar illumination for potential dual-use applications. With further design optimisation, it has been predicted that the conversion efficiency will exceed 50% at 1.55 µm and 1 kW/m2.

Efficiency limits of laser power converters for optical power transfer applications

We have developed III–V-based high-efficiency laser power converters (LPCs), optimized specifically for converting monochromatic laser radiation at the eye-safe wavelength of 1.55 µm into electrical power. The applications of these photovoltaic cells include high-efficiency space-based and terrestrial laser power transfer and subsequent conversion to electrical power. In addition, these cells also find use in fibre-optic power delivery, remote powering of subcutaneous equipment and several other optical power delivery applications. The LPC design is based on lattice-matched InGaAsP/InP and incorporates elements for photon-recycling and contact design for efficient carrier extraction. Here we compare results from electro-optical design simulations with experimental results from prototype devices studied both in the lab and in field tests. We analyse wavelength and temperature dependence of the LPC characteristics. An experimental conversion efficiency of 44.6% [±1%] is obtained from the prototype devices under monochromatic illumination at 1.55 µm (illumination power density of 1 kW m−2) at room temperature. Further design optimization of our LPC is expected to scale the efficiency beyond 50% at 1 kW m−2.

Acoustooptic Bragg diffraction without overemodulation with a phased-array transducer

Acoustooptic Bragg diffraction without overemodulation (i.e., with the extended high-efficiency region on the diffraction efficiency dependence vs. sound amplitude) is studied theoretically and experimentally. It is found that this effect appears in the case of a symmetrically nonuniform acoustic field and is due to the equality of additional opposite phase shifts of light beams passing through symmetric regions of the acoustic field. The situation is considered when an acoustic field is excited by a three-section phased-array transducer. The conditions are determined, in which reverse optical power transfer from the diffracted beam to transmitted beam (overemodulation) in the case of a high (close to 100%) diffraction efficiency is considerably suppressed. In the case of a phased array, the effect weakly depends on the frequency of sound and the size of transducer sections, which makes it possible to observe it in a wide range of acoustooptic interaction parameters.

All-optical demultiplexing of 4-ASK optical signals with four-wave mixing optical gates

A new all-optical technique to demultiplex quaternary-amplitude-shift keying (4-ASK) signals into two on–off keying (OOK) signals is proposed. We show that such quaternary-to-binary conversion may be performed with two types of optical gates (OGs); a first one that implements an S-shaped optical power transfer function (TF), which is also the characteristic TF of optical pulse reshaping devices, and a second type that implements a U-shaped TF. Furthermore, we describe a heuristic approach to design such OGs by using the fiber four-wave mixing effect in highly non-linear dispersion-shifted fibers. The performance of the technique is evaluated by simulating the demultiplexing of 4-ASK signals with these OGs. Results indicate that the proposed devices are able to provide binary signals with acceptable BERs even after the 4-ASK signals are propagated through a cascade of eight fiber links of 50 km followed by optical amplifiers with a 4.5-dB noise figure.

Wireless signal‐preamble assisted Mach–Zehnder modulator bias stabilisation in wireless signal transmission over optical fibre

AbstractLithium niobate based Mach–Zehnder electro‐optic modulators are increasingly being used in high‐speed digital as well as analog optical links. Depending on the application, digital or analog, the bias point of such a modulator is held constant at a particular point on the sinusoidal electrical to optical power transfer characteristics of the modulator. Bias point drift is one of the major limitations of lithium niobate based Mach–Zehnder electro‐optic modulators. This increases the bit error rate of the system and affects adjacent channel performances. In one of the most popular methods of bias control, a pilot tone is used to track the bias point drift. However, pilot tone based bias tracking reduces overall intermodulation free dynamic range of the link. In this paper we propose a method where Mach–Zehnder modulator bias drift is tracked and maintained at the desired point by tracking the power variation of the preamble of wireless signal data frames. The method has no detrimental effects on system performances as no external signal is exclusively injected into the system for bias tracking purposes. Copyright © 2007 John Wiley & Sons, Ltd.

Optical Power Transfer of Grating - Assisted Directional Coupler with Three - Guiding Channels : TM modes Case

Newly developed modal transmission-line theory(MTLT) is used to analyze rigorously the optical power distribution in grating-assisted directional couplers(GADCs) with three guiding channels. By defining a novel coupling efficiency ${\eta}$ amenable to the rigorous analytical solutions of modal transmission-line theory, we explicitly evaluate the power coupling and distribution of TM modes. The results reveal that the incident power is sensitively partitioned through three output channels in terms of such grating parameters as the grating period, the duty cycle, and the operating wavelength.

A novel and compact optical polarizer incorporating a layered waveguide core structure

A compact optical polarizer based on a directional coupler incorporating layered waveguide core structures is studied in detail in this paper. Important waveguide design parameters such as the guide width, guide separation, guide height, refractive index contrast, and wavelength dependence have been analyzed by using the vectorial H-field finite-element method. The optical power transfer characteristics have been calculated by using the least squares boundary residual methods.

Transmitted Optical Power through a Tapered Single-Mode Fiber under Dynamic Bending Effects

This paper describes theoretically one of the physical phenomena that take place in a tapered single-mode optical fiber (taper): the transfer of optical power between different order modes. This optical power transfer depends on several factors; one of them is the static bending of the taper, as other authors have reported. In addition to these studies, we have noticed that if the bend angle oscillates, then the magnitude of the optical power transfer is also modified by other additional factors. Thus, at dynamic conditions the transmitted optical power through the taper is also affected by the oscillation frequency. Here we propose a more generic equation system to obtain the coupling coefficient between modes under static as well as dynamic bending conditions. When the presented equations are particularized for static conditions, a complete agreement with other studies is observed. In addition, validating the theoretical justification, some experimental results are also shown.

Enhancement of power transfer in periodic array of optical waveguides via intermediate Bloch states

A directional coupling mechanism between different waveguides in a periodic array of waveguides is suggested. The optical power transfer between two different waveguides is mediated by the coupling between zero-order modes of two of the waveguides and the second band of the periodic structure. Analytical solutions for the no-detuning (narrow band) and far-from-resonance cases are presented. The far-from-resonance case is shown to resemble a simple two-mode system with complete optical power transfer between the two waveguides, coupled by localized gratings. The transfer is mediated by the second band of the periodic structure. The transition length depends strongly on the shape of the perturbation, and depends exponentially on the distance between the waveguides, yet it allows us to transfer power from one waveguide to another at such distances, for which the transition via conventional directional tunneling mechanism is impossible. Our analytical results are supported by numerical calculations carried out for a model problem with realistic parameters.

측면 연마 광섬유 결합기를 이용한 파장분할 다중화 채널분리 필터

파장분할 다중화 시스템의 다중 채널분리 필터가 측면연마 광섬유 결합기를 이용한 빗살필터로 구현됨을 제안하였다 단일 모드 광섬유의 기본모드와 결합하는 상부 평면 도파로의 모드가 고차일수록 광 전력전달 효율은 낮아지는데, 두 도파로 사이 에 중간 결합층을 삽입함으로서 개선시킬 수 있고 최적화 조건에서 20dB 이상의 채널간 소멸비를 얻을 수 있음을 보였다. 200fm두께의 리튬나오베이트를 상부 평면 도파로로 적용한 경우 4m의 채널간격을 가지는 빗살필터 특성을 얻었으며, 중간 결합층의 두께가 1 Um일 때 굴절률이 1.52-1.53 범위에서 최대 광 전력전달이 일어남을 관측하였고 빔 전파 방법을 이용하여 계산한 결과와 거의 일치함을 보였다. Fiber-optic comb filters using a side-polished fiber coupler are proposed as multi-channel isolation filters on wavelength division multiplexing systems. We have demonstrated that the coupling efficiency between two waveguides is improved by the intermediate coupling layer in spite of the decrease of the optical power transfer between two waveguides due to the high-order modes of the overlay waveguide coupled with the side-polished single-mode fiber. When LiNbO<TEX>$_3$</TEX>with a 200-<TEX>${\mu}{\textrm}{m}$</TEX>-thickness was applied as a planar-overlay-waveguide, the comb filtering characteristics with a 4 nm-channel-spacing were achieved and the maximum power coupling occurred at the 1-<TEX>${\mu}{\textrm}{m}$</TEX>-thickness and the refractive index in range 1.52 to 1.53 of an intermediate coupling layer. If the intermediate coupling layer is optimized, an extinction ratio with more than 20 dB can be obtained. These experimental results are in good agreement with the BPM simulation.

Non-evanescent adiabatic directional coupler

A directional coupling mechanism based on an adiabatic coupling between three optical modes is suggested. The optical power transfer between two waveguides which are far apart is mediated by adiabatic coupling between zero-order optical modes of the individual waveguides and a high-order intermediate mode. The analytical model for an adiabatic three-mode coupling based on a scalar wave equation is presented. The directional coupling via the adiabatic mode coupling between copropagating modes is described and compared with a nonadiabatic directional coupling assisted by periodic perturbation. It is shown that adiabatic directional coupling has much less sensitivity to the mode parameters and to the wavelength.

Signal wavelength dependence of gain saturation in a fiber optical parametric amplifier

Signal gain-saturation characteristics in a fiber optical parametric amplifier are described. Experiments and calculations for the high-gain-saturation region are presented in which the direction of optical power transfer, which is from the pump to the signal and the idler in the unsaturated region, becomes reversed at high-signal-input powers. It is shown that the signal gain soon saturates for signal wavelengths far from the pump. This wavelength dependence is explained by the power-dependent phase mismatch and its behavior in the high-signal-input region.

Highly efficient optical power transfer to whispering-gallery modes by use of a symmetrical dual-coupling configuration

We report that greater than 99.8% optical power transfer to whispering-gallery modes was achieved in fused-silica microspheres by use of a dual-tapered-fiber coupling method. The intrinsic cavity loss and the taper-to-sphere coupling coefficient are inferred from the experimental data. It is shown that the low intrinsic cavity loss and the symmetrical dual-coupling structure are crucial for obtaining the high coupling efficiency.

Suppression of Signal Power Depletion by Filtering of FWM Induced Conjugate Waves in Repeatedly Loss Compensated System

Optical power transfer due to four-wave mixing (FWM) in repeatedly loss compensated systems is studied, This optical power transfer limits long-haul wavelength division multiplexing systems which employ the unequally spaced channel allocation technique. It is confirmed that the filtering out of conjugate lights can effectively suppress the optical power transfer.

High-power high-frequency traveling-wave heterojunction phototransistors with integrated polyimide waveguide

A high-power high-speed phototransistor has been demonstrated using a traveling-wave (TW) structure with an integrated polyimide optical waveguide. In our configuration, optical power transfer is distributed along the length of the device via leaky mode coupling of light from the polyimide waveguide to the active region of the phototransistor. The TW electrode design allows for an electrically long structure while maintaining high bandwidths. Due to the increased absorption volume, the optical power handling capabilities of the TW-heterojunction phototransistors (TW-HPT's) are improved over that of conventional lumped-element HPT detectors. The experimental results show no saturation of the fundamental at 60 GHz up to 50 mA of DC photocurrent.

Heating mechanisms in a near-field optical system

A finite-difference-time-domain and two finite-difference-thermal models are used to study various heating mechanisms in a near-field optical system. It is shown that the dominant mechanism of sample heating occurs from optical power that is transferred from the probe to a metallic thin-film sample. The optical power is absorbed in the sample and converted to heat. The effects of thermal radiation from the probe 's coating and thermal conduction between the probe and the sample are found to be negligible. In a two-dimensional waveguide with TE polarization, most of the optical power is transferred directly from the aperture to the sample. In a two-dimensional waveguide with TM polarization, there is significant optical power transfer between the probe 's aluminum coating and the sample. The power transfer results in a wider thermal distribution with TM polarization than with TE polarization. Using computed temperature distributions in a Co -Pt film, we predict the relative size of thermally written marks in a three-dimensional geometry. The predicted mark size shows a 30 % asymmetry that is due to polarization effects.

Integration of polyimide waveguides with traveling-wave phototransistors

Polymer waveguides with low optical losses are becoming increasingly important for photonic circuits. Among these, polyimide waveguides have properties which distinguish them as a versatile interconnect medium for practical optoelectronic integrated circuits. We have integrated these waveguides with traveling-wave phototransistors (TW-HPT's) in which optical power transfer occurs through leaky mode coupling of light from the waveguide along the length of the device. Independent optimization of the waveguide and phototransistor is now possible permitting superior coupling efficiency and detector bandwidth. We present proof-of-concept results for a 2-mm-long TW-HPT integrated with a 8×10-μm polyimide waveguide.

Near field measurements of optical channel waveguides and directional couplers

Near field microscopy is used to investigate the guided mode intensity distribution of optical channel waveguides and directional couplers with subwavelength spatial resolution. The directional coupler consisted of two single mode optical ridge channel waveguides formed with silicon nitride deposited on a lower cladding layer of SiO2 on a silicon substrate. A near field measurement of the guided mode intensity profile in the transverse direction parallel to the waveguide surface was performed across one of the optical channel waveguides. These variations are compared with model calculations. Similar transverse measurements of light propagating through a directional coupler were performed at many locations along the coupler, providing a view of the evolution of optical power transfer.